MIL-STD-1553b is a 30-year-old standard that defines electrical and signaling characteristics for communications over avionics buses used in military and civilian aircraft, as well as in other applications (ships, trains, shuttles, space stations, etc.). A Manchester II bi-phase signaling scheme is used over shielded twisted pair cabling. That signaling scheme does not efficiently utilize potential bandwidth available on the bus.
OFDM is a communications protocol that may be used to more optimally utilize the available bandwidth unused by the 1553b signaling. Of course, bus coupler type, network topology and filtering of the Manchester II signaling affect how much bandwidth is available for an “overlay” OFDM communications system.
An OFDM based communications system can be described by transmitter 10 and receiver 30 components shown in FIGS. 1 and 3. The transmitter 10 includes forward error correction (FEC) 12 applied to an input data bit stream, followed by a mapping 14 of encoded bits to frequency domain sub-carriers, which are transformed to a time domain digital signal by an inverse fast Fourier transform (IFFT) 16, which is an efficient implementation of an inverse discrete Fourier transform (IDFT). FEC 12 may be a Reed-Solomon, convolutional, or any other type of forward error correction encoding scheme.
Before the digital signal is converted to an analog signal for transmission to the receiver 30, a preamble, inserted by preamble insertion 18, includes a number of synchronization symbols 24 (shown in FIG. 2) which are pre-pended to the transmission sequence to permit synchronization of the transmitted waveform at the receiver 30, and to facilitate automatic gain control (AGC) and channel response estimation. A cyclic prefix is usually added to the OFDM symbols, which are appropriately shaped (windowed and/or filtered) by symbol shaping 20 before conversion to an analog signal by an analog front end (AFE) 22. The AFE 22 includes a digital-to-analog converter (DAC), appropriate analog filtering and may also include an IF/RF mixing stage to convert the signal to higher frequencies.
An exemplary OFDM transmission sequence is shown in FIG. 2. As can be seen, a predetermined number of synchronization symbols 24 are prepended to data symbols 26.
At the receiver 30, the analog signal is filtered and converted to a digital signal by an analog-to-digital converter (ADC), not shown, in a receiver analog front end 32. The appropriate RF/IF stages are used to convert the received signal to a baseband signal in a manner well known in the art. An automatic gain control 34 controls input signal level based on power metrics estimated from the synchronization symbols 24. A fast Fourier transform (FFT) which is an efficient implementation of the discrete Fourier transform (DFT) 36 is applied to the sampled signal, with the timing of the FFT based on detection and timing estimations derived from the synchronization symbols 24. A channel estimation 46 is calculated using the synchronization symbols detected by the synchronization detection unit 44 and is used by the demodulator 38 to remove effects of the channel. This process, called channel equalization, is performed in the frequency domain. An inverse mapping function 40 is used to convert demodulated frequency domain sub-carriers to coded data bits followed by forward error correction (FEC) decoding 42, which corrects bit errors when possible and passes the decoded data bits to higher communications layers.
In addition to using OFDM to utilize unused bandwidth on a 1553b bus, it has been recognized as desirable to be able to configure multiple independent networks on the same bus, so that groups of communications devices can be respectively allocated a certain proportion of the available spectrum. It would also be useful for some devices (a host bus controller, for example) to be able to communicate to devices associated with any of these independent networks. In order to accomplish these objectives, the synchronization signaling component of an OFDM-based communications system must have an efficient implementation and be configurable “on the fly”, as well as having other required properties. These properties include: a small transmit peak to average power for all configurations; and cross-correlation properties such that a receiver configured to operate in one frequency band will not detect as a valid synchronization signal leakage energy of a transmitter configured to operate in another frequency band.
Linear FM (LFM) signals are used in communications systems for the transmission of data as well as for synchronization preambles, automatic gain control (AGC) and channel response estimation. Advantages of LFM signals include low peak to average power for transmission using limited resolution digital to analog converters and limited linearity amplifiers, and having a narrow correlation peak for matched filter reception.
OFDM multi-carrier communications schemes, such as HomePlug® Version I and AV, often synthesize a LFM signal by storing frequency domain coefficients in a look up table (LUT) and then transforming to the time domain using an IDFT. Each LUT coefficient corresponds to an OFDM sub-carrier and each coefficient has a non-zero value for sub-carriers ranging over the sweep of the LFM signal. This system can be configured to sweep over any desired sub-band by zeroing LUT coefficients corresponding to sub-carriers outside the sub-band. However this system suffers from the drawback that when configured for a sub-band, the time domain LFM sweep will have most of its power concentrated in the time segment corresponding to the sub-band, significantly increasing the peak to average power ratio of the signal. For example, if the system is configured to sweep the first half of the LFM band by zeroing the upper half of the coefficients, the first half of the LFM waveform will sweep over this sub-band and the second half will be close to zero in amplitude. A bandwidth-configurable OFDM modem that overcomes at least one of these shortcomings would be highly desirable.